The invention relates to screw compressors, and more particularly to axial unloading lift valves for screw compressors.
Axial unloading lift valves are commonly used in screw compressors to vary the compression load produced by the screws. One or more valves are arranged axially towards the discharge side of the screws and the load is varied by selectively opening and closing the valves. Opening the valves to xe2x80x9cunloadxe2x80x9d the compressor reduces the effective working length of the screws by opening communication pathways between portions of the screw and the low-pressure suction end of the compressor. The open pathways allow the pressure to equalize so that compression does not occur over the portions of the screw communicating with the suction end of the compressor. When the valves are closed to xe2x80x9cloadxe2x80x9d the compressor, no pressure equalization occurs over the axial length of the screw. Therefore, the full working length of the screws is utilized for compression. The angular location of the valves around the discharge ends of the screws determines how much of the axial working length of the screws is used or eliminated when the valves are closed or opened.
FIGS. 1 and 2 are schematic representations showing a portion of a prior art compressor 10 having an axial unloading lift valve 14. FIG. 1 shows the valve 14 in the loaded condition and FIG. 2 shows the valve 14 in the unloaded condition. The compressor 10 includes a pair of screws 15, 16 (only one is shown in FIGS. 1 and 2) mounted for rotation in a screw housing 22. The interior of the screw housing 22 defines a compression chamber 24 where the fluid is compressed by the screws 15, 16, as is understood by those skilled in the art. A discharge housing 26 supports the discharge end of the screws 15, 16 and is coupled to one end of the screw housing 22. A suction housing 30 supports the suction end of the screws 15, 16 and is coupled to the other end of the screw housing 22.
The axial unloading valve 14 typically includes a cylindrically-shaped valve member 34 housed in a valve chamber 38. The valve chamber 38 is formed in the discharge housing 26 so that one end of the valve chamber 38 communicates both with the compression chamber 24 and with a vent passageway 42. The vent passageway 42 is connected to a suction cavity 46 formed in the suction housing 30. The other end of the valve chamber 38 communicates with a high-pressure fluid supply that controls the positioning of the valve member 34. The high-pressure fluid supply is typically either high-pressure lubricating oil or refrigerant that has been discharged from the compressor.
To load the compressor 10, the valve 14 is closed by flooding the valve chamber 38 with high-pressure fluid. The fluid in the valve chamber 38 forces the valve member 34 toward the screw housing 22 until the valve member 34 abuts the screw housing 22, as shown in FIG. 1. When the valve member 34 is in the position shown in FIG. 1, there is no communication, and therefore no pressure equalization, between the suction cavity 46 and the compression chamber 24. Because there is no pressure equalization, the entire working length of the screws 15, 16 is utilized and maximum compression loading is generated by the compressor 10.
To unload the compressor 10, the valve 14 is opened by draining the fluid from the valve chamber 38. The high-pressure fluid in the compression chamber 24 forces the valve member 34 away from the screw housing 22, as shown in FIG. 2. When the valve member 34 is in the position shown in FIG. 2, the passageway 42 provides communication, and therefore pressure equalization, between the compression chamber 24 and the suction cavity 46. This pressure equalization reduces the effective working length of the screws 15, 16, thereby reducing the compression load generated by the compressor 10.
For the axial unloading valve 14 to function properly, the valve member 34 must be carefully manufactured and installed. FIG. 3 shows a prior art valve member 34 in greater detail. The valve member 34 is substantially cylindrical and includes opposing first and second axial sealing surfaces 50 and 54, respectively. A radial sealing and positioning surface 58 extends between the axial sealing surfaces 50 and 54.
With this symmetrical configuration, the valve member 34 could be installed in the valve chamber 38 in two ways. Therefore, both the first and the second axial sealing surfaces 50 and 54 must be machined to tight axial run-out tolerances to ensure that, regardless of how the valve member 34 is installed, proper axial sealing occurs when the valve 14 is closed. The term xe2x80x9crun-outxe2x80x9d is well-known to those in manufacturing and in this situation is generally understood to refer to the perpendicularity between a longitudinal axis 62 and each of the axial sealing surfaces 50 and 54. In addition to sealing concerns, the tight run-out tolerance ensures that no portion of the valve member 34 will interfere with the 5 screws 15, 16 when the compressor 10 is operating at full load (i.e., when the valve 14 is closed). This is especially important on compressors having small axial screw endplay with respect to the discharge housing 26. Maintaining the tight axial run-out tolerances requires expensive precision machining and, because both axial sealing surfaces 50 and 54 must be tightly toleranced, two separate machine setups are required for two separate precision machining operations. This significantly increases the manufacturing cost of the valve member 34.
One way to eliminate the need for two tightly-toleranced axial sealing surfaces 50, 54 on the valve member 34 is to change the design. FIG. 4 illustrates an alternative prior art valve member 66 that has only one axial sealing surface 70. Additionally, the radial sealing surface 74 and the radial positioning surface 78 are separate surfaces. This ensures that the valve member 66 can only be installed in one way, thereby eliminating the need for a second axial sealing surface with a tight run-out tolerance.
While only one precision machining setup is necessary for achieving the desired run-out tolerance on the single axial sealing surface 70, a separate machining operation is still required to form the radial positioning surface 78. This second operation need not be precision machining, but nonetheless requires a second machine setup. The two separate machine setups required to manufacture the different radial surfaces 74 and 78 can create tolerance stack-up problems and often mandate the use of a gasket 82 to prevent leakage. The use of the gasket 82 also adds to the cost of the compressor 10 and increases the number of parts that may require periodic replacement.
The present invention provides an improved valve member for an axial unloading lift valve. The improved valve member has only one axial sealing surface requiring a tight run-out tolerance. Therefore, only one machine setup is needed to produce the sealing surfaces of the improved valve member. Additionally, the valve member of the present invention includes features that facilitate proper assembly and ensure that the valve member is properly installed. No gaskets are required to seal the valve member. Thus, the valve member of the present invention provides a less-expensive and more reliable valve member than the prior-art valve members described above.
More specifically, the invention provides a screw compressor having a housing, a drive screw supported by the housing, and an idler screw supported by the housing. The drive screw and idler screw assembly have a low-pressure end and a high-pressure end. The drive screw, driven by an outside force, drives the idler screw, to which the drive screw is operably engaged. Rotation of the screws moves a fluid from the low-pressure end to the high-pressure end. The screw compressor further has at least one vent passageway with one end in fluid communication with the low-pressure region and a second end in selective fluid communication with the high-pressure end. In addition, the screw compressor has at least one valve having a valve member. Each valve member has a sealing surface, a non-sealing surface, and a radial sealing surface partially extending between the sealing surface and the non-sealing surface, the non-sealing surface having a recess. The valve is positioned such that the valve member is installable in a correct orientation and an incorrect orientation. When installed in the correct orientation the valve member is movable between a loaded position, at which the valve member substantially prevents flow from the high-pressure end to the low-pressure end, and an unloaded position, at which fluid passes from the high-pressure region through the vent passageway to the low-pressure region. When the valve member is installed in the incorrect orientation, the valve member provides a flow path from the high-pressure end through the vent passageway to the low-pressure end when in the loaded position and the unloaded position.
Other features and advantages of the invention will become apparent to those skilled in the art upon review of the following detailed description, claims, and drawings.